But the comment does not say "you must not be a developer". If the context had included a "the guy in front of me, also a new hire, was greeted as "so...you the new developer", then yes we would have something that might be sexism here.

But we do not have that kind of context. We only have a assumed intention on what the person meant by it. If we do such assumptions, we assume bad faith.

It could be that they hired a new web developer, also called web designers by many, or designers as short. If a back-end or sysadmin meet someone who is working on the front end, or even on a API structure, its fully plausible to call them "designers", be them male, female, or alien.

Simply put, the comment without any additional context to vilify the speaker, is not sexism. With some context, it might be, but in many others, it is not. Assuming good faith is to default at those context that do not mean sexism.